This invention relates to an improved version of a vectored thrust ducted propeller tail assembly, sometimes referred to as a ring tail, for rotary wing aircraft in which a shrouded propeller contained within the tail duct incorporates provisions for deflecting the propeller slip stream emerging from the tail duct transversely of the aircraft for developing the lateral thrust forces necessary to counteract rotor torque and establish directional yaw control of the aircraft. Although highly effective for conventional helicopter types, a vectored thrust ducted propeller tail assembly configuration is particularly effective for use in compound helicopters in which the rotor is unloaded in the high forward speed range with lift being provided by the fixed wing and forward propulsion being provided by the thrust of the shrouded propeller.
The invention of this application is directed to improving the maneuvering agility of ring tail rotary wing aircraft, particularly yaw agility, which is of great importance in military rotary wing aircraft in which the aircraft must be turned to the proper heading in aiming its armament. Under combat conditions the ability to quickly change heading to the direction in which the armament must be aimed is often critical, hence military requirements necessitate military aircraft being able to establish an angular turning acceleration that will turn the aircraft 180 degrees in either direction within a few seconds under all flight conditions including those involving low rotor shaft torque and a power off mode of autorotation. Also, combat maneuvering agility is greatly improved by an ability to establish a high rate of acceleration or deceleration of aircraft speed along the line of flight.
Known shrouded propeller aircraft configurations, such as those disclosed in U.S. Pat. Nos. 3,222,012 and 3,260,482 and the latest configuration of U.S. Pat. No. 4,905,932, incorporate a combination of propeller slip stream deflecting surfaces at the rear of the tail duct as will deflect the emerging propeller slip stream of a required intensity in a direction that the resulting thrust establishes a rotor torque counterbalancing couple of the necessary magnitude to maintain the desired aircraft heading. The torque applied to an aircraft by the rotor being opposite the direction of rotor rotation, in a rotary wing aircraft having a counterclockwise rotating rotor of the nature disclosed in U.S. Pat. No. 4,905,932, the propeller slip stream is deflected to the left as it emerges from the exit side of the tail duct establishing a starboard thrust creating a rotor torque counterbalancing moment. The magnitude of the transversely directed thrust is established by the propeller pitch, this thrust provides yaw control as the thrust becomes equal to, greater than, or less than that required to counterbalance rotor torque. Initiating a counterclockwise or left turn for the counterclockwise rotating rotor aircraft of the type disclosed in U.S. Pat. No. 4,905,932 requires the propeller pitch be increased an amount as creates a starboard thrust greater than that required to counterbalance rotor torque. Similarly, a clockwise or right turn requires the propeller pitch be decreased to the degree that the generated starboard thrust is less than that required to counterbalance rotor torque. Therefore, the maximum rate of rotation that can be generated for a clockwise turn to the right is dependent upon the amount of rotor shaft torque that is being created to sustain the aircraft in flight under existing flight conditions. For certain flight conditions the amount of rotor shaft torque might not be sufficiently great to establish the required angular turning acceleration that would be required for combat maneuverability. Obviously, the counterclockwise or left turning maneuverability of a clockwise rotating rotor ring tail aircraft encounters the same limitations. Therefore, to develop angular turning accelerations needed for certain combat situations while the aircraft is in a hover mode under certain flight conditions, turning torque additional to the torque being developed by the rotor shaft can be required.
Combat maneuverability for military rotary wing aircraft also involves an ability to rapidly accelerate or decelerate the forward motion of the aircraft. The ring tail configuration of shrouded propeller ring tail aircraft of the nature of those disclosed in the previously mentioned patents have the capability of rapid acceleration while in the forward high speed mode through rapidly increasing propeller pitch but forward speed deceleration is dependent upon aircraft drag which is a fixed amount and must be supplemented if a retardation rate of speed greater than that provided by the aircraft drag is required for combat maneuvering.